The present invention relates to a compound EDM apparatus having the capability of effecting travelling wire cutting operations and cavity sinking or shaping operations. Among the diverse methods available for machining a workpiece by electrical discharges, travelling wire cutting of a workpiece tends today to take the lead over cavity sinking machining by means of a shaping massive electrode tool. One of the reasons is due principally to the fact that travelling wire EDM machining is effected at a relatively low cost by an electrode tool, in the form of a wire, whose size has no influence upon the accuracy of machining. EDM die sinking, or cavity sinking, by contrast, requires an expensive and massive electrode tool that, in addition, must be periodically replaced or rectified, because of the unavoidable wear to which it is subjected, which in turn necessarily reduces the machining accuracy, whatever the precautions taken to control the wear of the electrode tool and the accuracy of machining. There results a tendency for the user of EDM apparatus to emphasize travelling wire cutting operations as much as possible, which obviously results in a limitation of workpiece shapes which can be obtained. However, a great number of diverse workpiece shapes could be obtained economically by travelling wire cutting of the workpiece followed by an additional cavity sinking or shaping permitting to machine particular areas of the workpiece with a specific shape, for example for forming recesses which can not be achieved by means of an electrode wire. As the massive electrode tool used in sinking operations can often be made by EDM wire cutting, a great number of machining operations, including the machining of the massive cavity sinking electrode tool could be entirely effected by means of two EDM apparatus, one of the travelling wire type which is used both for making the massive die or cavity sinking electrode tools and for effecting a preliminary cut in workpieces by means of the electrode wire, while the other EDM apparatus is of the die or cavity sinking type and is used for finish machining operations on the workpiece having been roughly cut on the first EDM machine, the electrode tool used on the second machine having also been machined on the first EDM machine.
One of the most expensive parts of a precision EDM machine, whether it is a travelling wire EDM machine or a cavity sinking machine, is the mechanical assembly permitting to effectuate with high accuracy the controlled displacement of the electrode wire or of the massive shaping electrode tool, as the case may be, relative to the workpiece. For some applications, it would therefore be advantageous to utilize the same mechanical assembly providing such relative displacement for the three types of machining mentioned above, namely machining of the massive form electrode tool for die or cavity sinking, effecting the preliminary cut in the workpiece, and effecting the finish machining of the workpiece by shaping or sinking by the massive form electrode tool.